It is known that, to achieve increasingly higher efficiency in gas turbines, in particular latest-generation ones, it is necessary both to use increasingly higher start-of-expansion temperatures and to obtain, in the most efficient way possible, an optimal homogeneity of temperature on the blades. Said results can be achieved and, in actual fact, are currently achieved, using combustion chambers with annular geometry.
The aforementioned combustion chambers enable excellent performance both as regards efficiency of combustion and as regards the limitation of pollutant emissions and the high density of thermal yield (MWth/m3). However, on the basis of the results of some verifications, it may be stated that the annular geometry associated to high densities of thermal yield can favour onset of phenomena of thermo-acoustic instability. The latter occur with marked oscillations of pressure within the combustion chamber, at well-defined frequencies that are characteristic of the geometry of the combustor and of the running conditions. Said oscillations can bring about undesirable vibrations in the turbine and damage its components.
To limit this problem, manufacturers of gas turbines have developed various techniques.
Some techniques are based upon decoupling of the forcing frequencies, generated by the peculiarities of the burner, from the natural frequencies of the mechanical system that enters into vibration. Other techniques are based upon control of the fuel in phase opposition with the onset of the pressure oscillations (active control). However, these methods, which are prevalently of an active type, have moving members and/or need to undergo operations of control and adjustment during the operating cycle of the gas turbine.
Also known are passive-damping systems, based upon the use of dissipater devices, in particular Helmholtz resonators, which capture the acoustic waves and dampen their amplitude, dissipating the energy thereof.
For example, the U.S. Pat. No. 6,530,221 relates to a system in which the dissipaters used are not Helmholtz resonators, but perforated box-section elements. A dissipater element of this type can give rise to the following problems:    1) the blades of the turbine may suffer damage in the case where one of the box-section elements is damaged on account of vibrations; and    2) application of the box-section elements is possible only on combustors of a cannular type and not on annular ones, in so far as, in the solution provided by the patent, the resonator is mounted on the can.
The U.S. Pat. No. 6,530,221 describes the use of a resonator device for application of which it is necessary to redesign the air chamber (i.e., a casing which surrounds the combustion chamber and delivers thereto the air for supporting combustion) and the combustion chamber. The mechanism for regulating the volume of the resonator proves moreover very delicate.
The British patent application GB 2 288 660 A describes a system in which the resonators used are classic Helmholtz resonators, sized according to relations available in the literature. However, the position in which the resonators should be mounted on the combustion chamber to be effective is not clarified. Furthermore, the volume of the resonator is not adjustable, so that the operating frequency is fixed. In order to overcome this drawback, the resonators are provided with a complicated system for regulation of the internal temperature so as to be able to regulate the frequency according to the temperature. In theory, the system is flexible, but at the expense of complications in terms of plant design and instrumentation, which limits the reliability thereof in an environment that is particularly critical, as regards temperature and pressure, as is that of a gas turbine.
Finally, the European patent application No. 0 597 138 A1 describes the application of a Helmholtz resonator to an annular combustion chamber, said resonator being mounted on the side of the combustion chamber (“upstream” portion or “front plate”) that carries the burner or burners. Hereinafter, the terms “upstream” and “downstream” are intended as referring to the direction of flow of the burnt gases in the combustion chamber.
Also in this case, the volume of the resonator is not adjustable, so that the operating frequency is fixed. Consequently, if the range of frequencies in which the resonator is effective is very restricted, as proves likely from the drawings (a range which, however, in this document is not defined, even indirectly), the damping could be insufficient in various operating conditions. Furthermore, the position of installation chosen for the resonator, as has been experimentally found by the technicians of the present applicant, is not the optimal position for its operation. In addition, for reasons of encumbrance, application of the resonator in the way indicated in EP 0597138A1 is not possible on combustion chambers different from the one hypothesized: for example, in the case of the majority of known turbines it would be necessary to redesign the air chamber and the combustion chamber.
Finally, it is to be highlighted that all the known solutions described above do not define the range of frequencies in which the resonator is effective, nor the effectiveness of damping of the pressure waves. Consequently, the state of the art that illustrates the application of passive resonators/dampers to combustion chambers of gas turbines in practice merely provides nothing but speculations as regards the possible effectiveness of the solutions proposed, without in effect providing to the person skilled in the branch any indication supported by experimental findings.